TL:DR; I'm upset about the new changes and have a lot of good reasons why. I want the old one back.

I'll begin by expressing that I absolutely despise this new user interface. I don't know what Three Rings was thinking. I've also not been playing for a while, so I have come back to a new and unexpected interface. As someone who broods over UIs and the importance of what they convey, I was not pleased.

First off, there have been major shifts in the design of user interfaces across most of computing. Skeuomorphism is being abandoned for a reason: It looks jarring and unappealing to a majority of users, myself included. While I would argue that Windows 7 handles skeuomorphism fairly well (not too much of it going on, very subtle and when necessary), things like the iOS interface is loathed by users everywhere by being overdone. Obviously with Linux it's completely the users's choice. Don't get me wrong, gradients are a good thing to use, but in modesty. (Win8 depresses me though.)

Spiral Knights had a good thing going, with the flat and clean layout. It didn't look dirty, morphed into something, and had a large amount of charm. I don't know what internal restaffing happened, or who decided that since there wasn't enough work to do elsewhere, redesigning the entire UI was a good idea, but it has been absolutely destroyed and made to look like plastic rather than a digital interface that matched the design of the Knights' faces. The staff internally refers to them as Tech Knights. Tech Knights indeed...

Secondly, the location of all the elements is all messed up. The health was originally in a very ideal location. I understand that moving the player's health to the upper left corner makes all other players' health visible at the same time while doing a self status check, but this detracts from the fact that monitoring one's own health has now become a horribly inefficient thing. Your eyes are centered on either yourself or something very nearby you at all times (except for when you're going for that long shot or running down an empty hallway). It takes fewer muscles and a lot less distance to glance down at the center of your screen for a self-status check.
There is now a shield bar for whatever inexplicable reason. The colors of your shield were more than enough to indicate when it was near breaking. I am a strong proponent of elements being visible on the character, and moving as much information into the environment to be read rather than on a bar. I get that you have both now, but the bar is distracting when doing a status-check, especially with the color choice. Which leads to my next point, which I will get to in a minute.
The location of the menu buttons is a bit odd now. Before, the menu would be a sort of dropdown, based on what you clicked. The button was at the top, and gave quick mouse access to the higher part of the window, which was typically the more important bits.
A personal gripe comes with the shift in location of the items/hotkey bar. I suppose it's fine on the bottom, but later down in this post, I'll try and make a point about it.

Other UI element design choices, starting with the color and length of the shield bar, are abysmal. Unless you have a lot of health and are at end-game, the shield bar is the first thing that draws your attention rather than your health. The sharp contrast of the blue against the red (not in the traditional color wheel sense, mind you. I know blue opposes yellow.) is jarring, with nothing physically separating them. Most games handle this by making both bars the same length permanently, and then just scaling the amount of points that fit into the bar. Other games handle this problem (like kingdom hearts) by making the bars both have an equal oportunity to scale, and in some cases games will do what KH does as well as make the lesser important bars MUCH thinner and a bit more of a pale color as not to draw the attention as quickly. (Kingdom Hearts had the odd challenge of balancing the meaning between both bars, as the player could inexplicably shift builds at any time, assuming their startup choice was balanced.)
The health indicator is now an actual bar, but for some reason keeps the old design of the pez icons. This conveys a very confusing message for me, and makes me wonder why the developers would expose sub-health-point granularity instead of keeping it as something that's a bit more mysterious. Individual health points that are fewer in number, yet larger in meaning, are a lot easier to track, and again add to the charm of the game. Harkening back to the old Zelda designs of individual hearts with only half-heart granularity, the game initially drew me in with these elements. I literally thought, "oh, awesome, a WindWaker artstyle in an MMO!" and was ecstatic that the developers were also big Nintendo fans. Your own opinions may vary of the company, but a lot of people playing this game also probably grew up playing a lot of Nintendo games.
And back to the menu buttons. I wasn't aware that we were shooting for World of Warcraft here. WoW had the exact same button design for the menu in the lower part of the screen. Tall, thin... pez-like shapes with little icons all standing in a row. Kinda sounds like an old health-indication system I used to love. I suppose you can't gripe too much about this, except that they need to be smaller. They take up a much larger screen real-estate than the old versions of the buttons did. I guess you can now fit more buttons here, but I'm burnt out on all these WoW clones.
Speaking of WoW clones, how many games have you seen put a hotbar of skills across the bottom before (As mentioned previously, I'm going to be making a point of this. Bear with me, as it's a bit of an aesthetics thing too)? Pretty much all of them. How many games have the little chicklet-shaped icons, with some kind of border, and are all minorly separated from eachother? Pretty much every MMO that came after WoW. Except maybe Guild-Wars 2, and they only pull ahead in the icon border decorations (there are none), although it is refreshing to see the flat, artsy design. I can think of another game that USED to have a unique look to the hotbar, but that's just a fading memory. If you're shooting for another World of Warcraft clone, then by all means go for it. I, for one, will not partake. Psychology is a powerful thing, and by dredging up memories of long hours dropped into a game that I don't want to ever return to (by saying "I" here, I intend to be generally inclusive of the gaming audience), you aren't doing yourselves any favors. Mainstreaming a game is an important thing to get more players, but I really don't think that this is not how it's done.

I could go on a bit more about this, but I'm a bit worn out just nitpicking the main screen. I might go on later about actual menus and that new pause screen, but for now I think I've expressed my opinion well enough.

My suggestion, as the title suggests, is to either make the old UI optional, or completely revert to it.